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tls E. Rescorla
Internet-Draft RTFM, Inc.
Intended status: Standards Track K. Oku
Expires: 19 April 2021 Fastly
N. Sullivan
C.A. Wood
Cloudflare
16 October 2020
TLS Encrypted Client Hello
draft-ietf-tls-esni-08
Abstract
This document describes a mechanism in Transport Layer Security (TLS)
for encrypting a ClientHello message under a server public key.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 19 April 2021.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Simplified BSD License text
as described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 4
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Topologies . . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Encrypted ClientHello (ECH) . . . . . . . . . . . . . . . 6
4. Encrypted ClientHello Configuration . . . . . . . . . . . . . 7
4.1. Configuration Extensions . . . . . . . . . . . . . . . . 8
5. The "encrypted_client_hello" Extension . . . . . . . . . . . 9
5.1. Encoding the ClientHelloInner . . . . . . . . . . . . . . 10
5.2. Authenticating the ClientHelloOuter . . . . . . . . . . . 11
6. Client Behavior . . . . . . . . . . . . . . . . . . . . . . . 11
6.1. Sending an Encrypted ClientHello . . . . . . . . . . . . 12
6.2. Recommended Padding Scheme . . . . . . . . . . . . . . . 14
6.3. Handling the Server Response . . . . . . . . . . . . . . 15
6.3.1. Accepted ECH . . . . . . . . . . . . . . . . . . . . 15
6.3.2. Rejected ECH . . . . . . . . . . . . . . . . . . . . 15
6.3.3. HelloRetryRequest . . . . . . . . . . . . . . . . . . 17
6.4. GREASE Extensions . . . . . . . . . . . . . . . . . . . . 18
7. Server Behavior . . . . . . . . . . . . . . . . . . . . . . . 18
7.1. Client-Facing Server . . . . . . . . . . . . . . . . . . 19
7.1.1. HelloRetryRequest . . . . . . . . . . . . . . . . . . 20
7.2. Backend Server Behavior . . . . . . . . . . . . . . . . . 21
8. Compatibility Issues . . . . . . . . . . . . . . . . . . . . 21
8.1. Misconfiguration and Deployment Concerns . . . . . . . . 22
8.2. Middleboxes . . . . . . . . . . . . . . . . . . . . . . . 22
9. Compliance Requirements . . . . . . . . . . . . . . . . . . . 23
10. Security Considerations . . . . . . . . . . . . . . . . . . . 23
10.1. Security and Privacy Goals . . . . . . . . . . . . . . . 23
10.2. Unauthenticated and Plaintext DNS . . . . . . . . . . . 24
10.3. Client Tracking . . . . . . . . . . . . . . . . . . . . 25
10.4. Optional Configuration Identifiers and Trial
Decryption . . . . . . . . . . . . . . . . . . . . . . 25
10.5. Outer ClientHello . . . . . . . . . . . . . . . . . . . 25
10.6. Related Privacy Leaks . . . . . . . . . . . . . . . . . 26
10.7. Attacks Exploiting Acceptance Confirmation . . . . . . . 27
10.8. Comparison Against Criteria . . . . . . . . . . . . . . 27
10.8.1. Mitigate Cut-and-Paste Attacks . . . . . . . . . . . 27
10.8.2. Avoid Widely Shared Secrets . . . . . . . . . . . . 28
10.8.3. Prevent SNI-Based Denial-of-Service Attacks . . . . 28
10.8.4. Do Not Stick Out . . . . . . . . . . . . . . . . . . 28
10.8.5. Maintain Forward Secrecy . . . . . . . . . . . . . . 28
10.8.6. Enable Multi-party Security Contexts . . . . . . . . 28
10.8.7. Support Multiple Protocols . . . . . . . . . . . . . 29
10.9. Padding Policy . . . . . . . . . . . . . . . . . . . . . 29
10.10. Active Attack Mitigations . . . . . . . . . . . . . . . 29
10.10.1. Client Reaction Attack Mitigation . . . . . . . . . 29
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10.10.2. HelloRetryRequest Hijack Mitigation . . . . . . . . 30
10.10.3. ClientHello Malleability Mitigation . . . . . . . . 31
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 33
11.1. Update of the TLS ExtensionType Registry . . . . . . . . 33
11.2. Update of the TLS Alert Registry . . . . . . . . . . . . 33
12. ECHConfig Extension Guidance . . . . . . . . . . . . . . . . 33
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 33
13.1. Normative References . . . . . . . . . . . . . . . . . . 33
13.2. Informative References . . . . . . . . . . . . . . . . . 34
Appendix A. Alternative SNI Protection Designs . . . . . . . . . 35
A.1. TLS-layer . . . . . . . . . . . . . . . . . . . . . . . . 35
A.1.1. TLS in Early Data . . . . . . . . . . . . . . . . . . 35
A.1.2. Combined Tickets . . . . . . . . . . . . . . . . . . 36
A.2. Application-layer . . . . . . . . . . . . . . . . . . . . 36
A.2.1. HTTP/2 CERTIFICATE Frames . . . . . . . . . . . . . . 36
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 36
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 36
1. Introduction
DISCLAIMER: This is very early a work-in-progress design and has not
yet seen significant (or really any) security analysis. It should
not be used as a basis for building production systems.
Although TLS 1.3 [RFC8446] encrypts most of the handshake, including
the server certificate, there are several ways in which an on-path
attacker can learn private information about the connection. The
plaintext Server Name Indication (SNI) extension in ClientHello
messages, which leaks the target domain for a given connection, is
perhaps the most sensitive information unencrypted in TLS 1.3.
The target domain may also be visible through other channels, such as
plaintext client DNS queries, visible server IP addresses (assuming
the server does not use domain-based virtual hosting), or other
indirect mechanisms such as traffic analysis. DoH [RFC8484] and
DPRIVE [RFC7858] [RFC8094] provide mechanisms for clients to conceal
DNS lookups from network inspection, and many TLS servers host
multiple domains on the same IP address. In such environments, the
SNI remains the primary explicit signal used to determine the
server's identity.
The TLS Working Group has studied the problem of protecting the SNI,
but has been unable to develop a completely generic solution.
[RFC8744] provides a description of the problem space and some of the
proposed techniques. One of the more difficult problems is "Do not
stick out" ([RFC8744], Section 3.4): if only sensitive or private
services use SNI encryption, then SNI encryption is a signal that a
client is going to such a service. For this reason, much recent work
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has focused on concealing the fact that the SNI is being protected.
Unfortunately, the result often has undesirable performance
consequences, incomplete coverage, or both.
The protocol specified by this document takes a different approach.
It assumes that private origins will co-locate with or hide behind a
provider (reverse proxy, application server, etc.) that protects
sensitive ClientHello parameters, including the SNI, for all of the
domains it hosts. These co-located servers form an anonymity set
wherein all elements have a consistent configuration, e.g., the set
of supported application protocols, ciphersuites, TLS versions, and
so on. Usage of this mechanism reveals that a client is connecting
to a particular service provider, but does not reveal which server
from the anonymity set terminates the connection. Thus, it leaks no
more than what is already visible from the server IP address.
This document specifies a new TLS extension, called Encrypted Client
Hello (ECH), that allows clients to encrypt their ClientHello to a
supporting server. This protects the SNI and other potentially
sensitive fields, such as the ALPN list [RFC7301]. This extension is
only supported with (D)TLS 1.3 [RFC8446] and newer versions of the
protocol.
2. Conventions and Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. All TLS notation comes from [RFC8446],
Section 3.
3. Overview
This protocol is designed to operate in one of two topologies
illustrated below, which we call "Shared Mode" and "Split Mode".
3.1. Topologies
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+---------------------+
| |
| 2001:DB8::1111 |
| |
Client <-----> | private.example.org |
| |
| public.example.com |
| |
+---------------------+
Server
Figure 1: Shared Mode Topology
In Shared Mode, the provider is the origin server for all the domains
whose DNS records point to it. In this mode, the TLS connection is
terminated by the provider.
+--------------------+ +---------------------+
| | | |
| 2001:DB8::1111 | | 2001:DB8::EEEE |
Client <----------------------------->| |
| public.example.com | | private.example.com |
| | | |
+--------------------+ +---------------------+
Client-Facing Server Backend Server
Figure 2: Split Mode Topology
In Split Mode, the provider is not the origin server for private
domains. Rather, the DNS records for private domains point to the
provider, and the provider's server relays the connection back to the
origin server, who terminates the TLS connection with the client.
Importantly, service provider does not have access to the plaintext
of the connection.
In the remainder of this document, we will refer to the ECH-service
provider as the "client-facing server" and to the TLS terminator as
the "backend server". These are the same entity in Shared Mode, but
in Split Mode, the client-facing and backend servers are physically
separated.
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3.2. Encrypted ClientHello (ECH)
ECH allows the client to encrypt sensitive ClientHello extensions,
e.g., SNI, ALPN, etc., under the public key of the client-facing
server. This requires the client-facing server to publish the public
key and metadata it uses for ECH for all the domains for which it
serves directly or indirectly (via Split Mode). This document
defines the format of the ECH encryption public key and metadata,
referred to as an ECH configuration, and delegates DNS publication
details to [HTTPS-RR], though other delivery mechanisms are possible.
In particular, if some of the clients of a private server are
applications rather than Web browsers, those applications might have
the public key and metadata preconfigured.
When a client wants to establish a TLS session with the backend
server, it constructs its ClientHello as usual (we will refer to this
as the ClientHelloInner message) and then encrypts this message using
the public key of the ECH configuration. It then constructs a new
ClientHello (ClientHelloOuter) with innocuous values for sensitive
extensions, e.g., SNI, ALPN, etc., and with an
"encrypted_client_hello" extension, which this document defines
(Section 5). The extension's payload carries the encrypted
ClientHelloInner and specifies the ECH configuration used for
encryption. Finally, it sends ClientHelloOuter to the server.
Upon receiving the ClientHelloOuter, the client-facing server takes
one of the following actions:
1. If it does not support ECH, it ignores the
"encrypted_client_hello" extension and proceeds with the
handshake as usual, per [RFC8446], Section 4.1.2.
2. If it supports ECH but cannot decrypt the extension, then it
terminates the handshake using the ClientHelloOuter. This is
referred to as "ECH rejection". When ECH is rejected, the server
sends an acceptable ECH configuration in its EncryptedExtensions
message.
3. If it supports ECH and decrypts the extension, it forwards the
ClientHelloInner to the backend, who terminates the connection.
This is referred to as "ECH acceptance".
Upon receiving the server's response, the client determines whether
or not ECH was accepted and proceeds with the handshake accordingly.
(See Section 6 for details.)
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Informally, a primary goal of ECH is ensuring that connections to
servers in the same anonymity set are indistinguishable from one
another without affecting any existing security properties of TLS
1.3. See Section 10.1 for more details about the ECH security and
privacy goals.
4. Encrypted ClientHello Configuration
ECH uses draft-05 of HPKE for public key encryption
[I-D.irtf-cfrg-hpke]. The ECH configuration is defined by the
following "ECHConfigs" structure.
opaque HpkePublicKey<1..2^16-1>;
uint16 HpkeKemId; // Defined in I-D.irtf-cfrg-hpke
uint16 HpkeKdfId; // Defined in I-D.irtf-cfrg-hpke
uint16 HpkeAeadId; // Defined in I-D.irtf-cfrg-hpke
struct {
HpkeKdfId kdf_id;
HpkeAeadId aead_id;
} ECHCipherSuite;
struct {
opaque public_name<1..2^16-1>;
HpkePublicKey public_key;
HpkeKemId kem_id;
ECHCipherSuite cipher_suites<4..2^16-4>;
uint16 maximum_name_length;
Extension extensions<0..2^16-1>;
} ECHConfigContents;
struct {
uint16 version;
uint16 length;
select (ECHConfig.version) {
case 0xfe08: ECHConfigContents contents;
}
} ECHConfig;
ECHConfig ECHConfigs<1..2^16-1>;
The "ECHConfigs" structure contains one or more "ECHConfig"
structures in decreasing order of preference. This allows a server
to support multiple versions of ECH and multiple sets of ECH
parameters.
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The "ECHConfig" structure contains the following fields:
version The version of ECH for which this configuration is used.
Beginning with draft-08, the version is the same as the code point
for the "encrypted_client_hello" extension. Clients MUST ignore
any "ECHConfig" structure with a version they do not support.
length The length, in bytes, of the next field.
contents An opaque byte string whose contents depend on the version.
For this specification, the contents are an "ECHConfigContents"
structure.
The "ECHConfigContents" structure contains the following fields:
public_name The non-empty name of client-facing server, i.e., the
entity trusted to update these encryption keys. This is used to
repair misconfigurations, as described in Section 6.3.
public_key The HPKE public key used by the client to encrypt
ClientHelloInner.
kem_id The HPKE KEM identifier corresponding to "public_key".
Clients MUST ignore any "ECHConfig" structure with a key using a
KEM they do not support.
cipher_suites The list of HPKE AEAD and KDF identifier pairs clients
can use for encrypting ClientHelloInner.
maximum_name_length The largest name the server expects to support,
if known. If this value is not known it can be set to zero, in
which case clients SHOULD use the inner ClientHello padding scheme
described below. That could happen if wildcard names are in use,
or if names can be added or removed from the anonymity set during
the lifetime of a particular resource record value.
extensions A list of extensions that the client must take into
consideration when generating a ClientHello message. These are
described below (Section 4.1).
4.1. Configuration Extensions
ECH configuration extensions are used to to provide room for
additional functionality as needed. See Section 12 for guidance on
which types of extensions are appropriate for this structure.
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The format is as defined in [RFC8446], Section 4.2. The same
interpretation rules apply: extensions MAY appear in any order, but
there MUST NOT be more than one extension of the same type in the
extensions block. An extension can be tagged as mandatory by using
an extension type codepoint with the high order bit set to 1. A
client that receives a mandatory extension they do not understand
MUST reject the "ECHConfig" content.
Clients MUST parse the extension list and check for unsupported
mandatory extensions. If an unsupported mandatory extension is
present, clients MUST ignore the "ECHConfig".
5. The "encrypted_client_hello" Extension
The encrypted ClientHelloInner is carried in an
"encrypted_client_hello" extension, defined as follows:
enum {
encrypted_client_hello(0xfe08), (65535)
} ExtensionType;
The extension request is carried by the ClientHelloOuter, i.e., the
ClientHello transmitted to the client-facing server. The payload
contains the following "ClientECH" structure:
struct {
ECHCipherSuite cipher_suite;
opaque config_id<0..255>;
opaque enc<1..2^16-1>;
opaque payload<1..2^16-1>;
} ClientECH;
cipher_suite The cipher suite used to encrypt ClientHelloInner.
This MUST match a value provided in the corresponding
"ECHConfig.cipher_suites" list.
config_id The configuration identifier, equal to "Expand(Extract("",
config), "tls ech config id", Nh)", where "config" is the
"ECHConfig" structure and "Extract", "Expand", and "Nh" are as
specified by the cipher suite KDF. (Passing the literal """" as
the salt is interpreted by "Extract" as no salt being provided.)
The length of this value SHOULD NOT be less than 16 bytes unless
it is optional for an application; see Section 10.4.
enc The HPKE encapsulated key, used by servers to decrypt the
corresponding "payload" field.
payload The serialized and encrypted ClientHelloInner structure,
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encrypted using HPKE as described in Section 6.1.
When offering the "encrypted_client_hello" extension in its
ClientHelloOuter, the client MUST also offer an empty
"encrypted_client_hello" extension in its ClientHelloInner, wherever
applicable. (This requirement is not applicable when the extension
is generated as described in Section 6.4.)
When the client offers the "encrypted_client_hello" extension, the
server MAY include an "encrypted_client_hello" extension in its
EncryptedExtensions message with the following payload:
struct {
ECHConfigs retry_configs;
} ServerECH;
retry_configs An ECHConfigs structure containing one or more
ECHConfig structures, in decreasing order of preference, to be
used by the client in subsequent connection attempts.
This document also defines the "ech_required" alert, which clients
MUST send when it offered an "encrypted_client_hello" extension that
was not accepted by the server. (See Section 11.2.)
5.1. Encoding the ClientHelloInner
Some TLS 1.3 extensions can be quite large and having them both in
ClientHelloInner and ClientHelloOuter will lead to a very large
overall size. One particularly pathological example is "key_share"
with post-quantum algorithms. In order to reduce the impact of
duplicated extensions, the client may use the "outer_extensions"
extension.
enum {
outer_extensions(0xfd00), (65535)
} ExtensionType;
ExtensionType OuterExtensions<2..254>;
OuterExtensions consists of one or more ExtensionType values, each of
which reference an extension in ClientHelloOuter.
When sending ClientHello, the client first computes ClientHelloInner,
including any PSK binders. It then computes a new value, the
EncodedClientHelloInner, by first making a copy of ClientHelloInner.
It then replaces the legacy_session_id field with an empty string.
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The client then MAY substitute extensions which it knows will be
duplicated in ClientHelloOuter. To do so, the client removes and
replaces extensions from EncodedClientHelloInner with a single
"outer_extensions" extension. Removed extensions MUST be ordered
consecutively in ClientHelloInner. The list of outer extensions,
OuterExtensions, includes those which were removed from
EncodedClientHelloInner, in the order in which they were removed.
Finally, EncodedClientHelloInner is serialized as a ClientHello
structure, defined in Section 4.1.2 of [RFC8446]. Note this does not
include the four-byte header included in the Handshake structure.
The client-facing server computes ClientHelloInner by reversing this
process. First it makes a copy of EncodedClientHelloInner and copies
the legacy_session_id field from ClientHelloOuter. It then looks for
an "outer_extensions" extension. If found, it replaces the extension
with the corresponding sequence of extensions in the
ClientHelloOuter. If any referenced extensions are missing or if
"encrypted_client_hello" appears in the list, the server MUST abort
the connection with an "illegal_parameter" alert.
The "outer_extensions" extension is only used for compressing the
ClientHelloInner. It MUST NOT be sent in either ClientHelloOuter or
ClientHelloInner.
5.2. Authenticating the ClientHelloOuter
To prevent a network attacker from modifying the reconstructed
ClientHelloInner (see Section 10.10.3), ECH authenticates
ClientHelloOuter by deriving a ClientHelloOuterAAD value. This is
computed by serializing ClientHelloOuter with the
"encrypted_client_hello" extension removed. ClientHelloOuterAAD is
then passed as the associated data parameter to the HPKE encryption.
Note the decompression process in Section 5.1 forbids
"encrypted_client_hello" in OuterExtensions. This ensures the
unauthenticated portion of ClientHelloOuter is not incorporated into
ClientHelloInner.
6. Client Behavior
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6.1. Sending an Encrypted ClientHello
To offer ECH, the client first chooses a suitable ECH configuration.
To determine if a given "ECHConfig" is suitable, it checks that it
supports the KEM algorithm identified by "ECHConfig.kem_id" and at
least one KDF/AEAD algorithm identified by "ECHConfig.cipher_suites".
Once a suitable configuration is found, the client selects the cipher
suite it will use for encryption. It MUST NOT choose a cipher suite
not advertised by the configuration.
Next, the client constructs the ClientHelloInner message just as it
does a standard ClientHello, with the exception of the following
rules:
1. It MUST NOT offer to negotiate TLS 1.2 or below. Note this is
necessary to ensure the backend server does not negotiate a TLS
version that is incompatible with ECH.
2. It MUST NOT offer to resume any session for TLS 1.2 and below.
3. It SHOULD contain TLS padding [RFC7685] as described in
Section 6.2.
4. If it intends to compress any extensions (see Section 5.1), it
MUST order those extensions consecutively.
The client then constructs EncodedClientHelloInner as described in
Section 5.1. Finally, it constructs the ClientHelloOuter message
just as it does a standard ClientHello, with the exception of the
following rules:
1. It MUST offer to negotiate TLS 1.3 or above.
2. If it compressed any extensions in EncodedClientHelloInner, it
MUST copy the corresponding extensions from ClientHelloInner.
3. It MAY copy any other field from the ClientHelloInner except
ClientHelloInner.random. Instead, It MUST generate a fresh
ClientHelloOuter.random using a secure random number generator.
(See Section 10.10.1.)
4. It MUST copy the legacy_session_id field from ClientHelloInner.
This allows the server to echo the correct session ID for TLS
1.3's compatibility mode (see Appendix D.4 of [RFC8446]) when ECH
is negotiated.
5. It MUST include an "encrypted_client_hello" extension with a
payload constructed as described below.
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6. The value of "ECHConfig.public_name" MUST be placed in the
"server_name" extension.
7. It MUST NOT include the "pre_shared_key" extension. (See
Section 10.10.3.)
The client might duplicate non-sensitive extensions in both messages.
However, implementations need to take care to ensure that sensitive
extensions are not offered in the ClientHelloOuter. See Section 10.5
for additional guidance.
To encrypt EncodedClientHelloInner, the client first computes
ClientHelloOuterAAD as described in Section 5.2. Note this requires
the "encrypted_client_hello" be computed after all other extensions.
In particular, this is possible because the "pre_shared_key"
extension is forbidden in ClientHelloOuter.
The client then generates the HPKE encryption context. Finally, it
computes the encapsulated key, context, HRR key (see Section 6.3.3),
and payload as:
pkR = Deserialize(ECHConfig.public_key)
enc, context = SetupBaseS(pkR,
"tls ech" || 0x00 || ECHConfig)
ech_hrr_key = context.Export("tls ech hrr key", 32)
payload = context.Seal(ClientHelloOuterAAD,
EncodedClientHelloInner)
Note that the HPKE functions Deserialize and SetupBaseS are those
which match "ECHConfig.kem_id" and the AEAD/KDF used with "context"
are those which match the client's chosen preference from
"ECHConfig.cipher_suites". The "info" parameter to SetupBaseS is the
concatenation of "tls ech", a zero byte, and the serialized
ECHConfig.
The value of the "encrypted_client_hello" extension in the
ClientHelloOuter is a "ClientECH" with the following values:
* "cipher_suite", the client's chosen cipher suite;
* "config_id", the identifier of the chosen ECHConfig structure;
* "enc", as computed above; and
* "payload", as computed above.
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If optional configuration identifiers (see Section 10.4)) are used,
the "config_id" field MAY be empty or randomly generated. Unless
specified by the application using (D)TLS or externally configured on
both sides, implementations MUST compute the field as specified in
Section 5.
6.2. Recommended Padding Scheme
This section describes a deterministic padding mechanism based on the
following observation: individual extensions can reveal sensitive
information through their length. Thus, each extension in the inner
ClientHello may require different amounts of padding. This padding
may be fully determined by the client's configuration or may require
server input.
By way of example, clients typically support a small number of
application profiles. For instance, a browser might support HTTP
with ALPN values ["http/1.1, "h2"] and WebRTC media with ALPNs
["webrtc", "c-webrtc"]. Clients SHOULD pad this extension by
rounding up to the total size of the longest ALPN extension across
all application profiles. The target padding length of most
ClientHello extensions can be computed in this way.
In contrast, clients do not know the longest SNI value in the client-
facing server's anonymity set without server input. For the
"server_name" extension with length D, clients SHOULD use the
server's length hint L (ECHCOnfig.maximum_name_length) when computing
the padding as follows:
1. If L >= D, add L - D bytes of padding. This rounds to the
server's advertised hint, i.e., ECHConfig.maximum_name_length.
2. Otherwise, let P = 31 - ((D - 1) % 32), and add P bytes of
padding, plus an additional 32 bytes if D + P < L + 32. This
rounds D up to the nearest multiple of 32 bytes that permits at
least 32 bytes of length ambiguity.
In addition to padding ClientHelloInner, clients and servers will
also need to pad all other handshake messages that have sensitive-
length fields. For example, if a client proposes ALPN values in
ClientHelloInner, the server-selected value will be returned in an
EncryptedExtension, so that handshake message also needs to be padded
using TLS record layer padding.
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6.3. Handling the Server Response
As described in Section 7, the server MAY either accept ECH and use
ClientHelloInner or reject it and use ClientHelloOuter. In handling
the server's response, the client's first step is to determine which
value was used. The client presumes acceptance if the last 8 bytes
of ServerHello.random are equal to "accept_confirmation" as defined
in Section 7.2. Otherwise, it presumes rejection.
6.3.1. Accepted ECH
If the server used ClientHelloInner, the client proceeds with the
connection as usual, authenticating the connection for the origin
server.
6.3.2. Rejected ECH
If the server used ClientHelloOuter, the client proceeds with the
handshake, authenticating for ECHConfig.public_name as described in
Section 6.3.2.1. If authentication or the handshake fails, the
client MUST return a failure to the calling application. It MUST NOT
use the retry keys.
Otherwise, when the handshake completes successfully with the public
name authenticated, the client MUST abort the connection with an
"ech_required" alert. It then processes the "retry_configs" field
from the server's "encrypted_client_hello" extension.
If one of the values contains a version supported by the client, it
can regard the ECH keys as securely replaced by the server. It
SHOULD retry the handshake with a new transport connection, using
that value to encrypt the ClientHello. The value may only be applied
to the retry connection. The client MUST continue to use the
previously-advertised keys for subsequent connections. This avoids
introducing pinning concerns or a tracking vector, should a malicious
server present client-specific retry keys to identify clients.
If none of the values provided in "retry_configs" contains a
supported version, the client can regard ECH as securely disabled by
the server. As below, it SHOULD then retry the handshake with a new
transport connection and ECH disabled.
If the field contains any other value, the client MUST abort the
connection with an "illegal_parameter" alert.
If the server negotiates an earlier version of TLS, or if it does not
provide an "encrypted_client_hello" extension in EncryptedExtensions,
the client proceeds with the handshake, authenticating for
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ECHConfigContents.public_name as described in Section 6.3.2.1. If an
earlier version was negotiated, the client MUST NOT enable the False
Start optimization [RFC7918] for this handshake. If authentication
or the handshake fails, the client MUST return a failure to the
calling application. It MUST NOT treat this as a secure signal to
disable ECH.
Otherwise, when the handshake completes successfully with the public
name authenticated, the client MUST abort the connection with an
"ech_required" alert. The client can then regard ECH as securely
disabled by the server. It SHOULD retry the handshake with a new
transport connection and ECH disabled.
Clients SHOULD implement a limit on retries caused by
"ech_retry_request" or servers which do not acknowledge the
"encrypted_client_hello" extension. If the client does not retry in
either scenario, it MUST report an error to the calling application.
6.3.2.1. Authenticating for the Public Name
When the server rejects ECH or otherwise ignores
"encrypted_client_hello" extension, it continues with the handshake
using the plaintext "server_name" extension instead (see Section 7).
Clients that offer ECH then authenticate the connection with the
public name, as follows:
* The client MUST verify that the certificate is valid for
ECHConfigContents.public_name. If invalid, it MUST abort the
connection with the appropriate alert.
* If the server requests a client certificate, the client MUST
respond with an empty Certificate message, denoting no client
certificate.
Note that authenticating a connection for the public name does not
authenticate it for the origin. The TLS implementation MUST NOT
report such connections as successful to the application. It
additionally MUST ignore all session tickets and session IDs
presented by the server. These connections are only used to trigger
retries, as described in Section 6.3. This may be implemented, for
instance, by reporting a failed connection with a dedicated error
code.
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6.3.3. HelloRetryRequest
If the server sends a HelloRetryRequest in response to the
ClientHello, the client sends a second updated ClientHello per the
rules in [RFC8446]. However, at this point, the client does not know
whether the server processed ClientHelloOuter or ClientHelloInner,
and MUST regenerate both values to be acceptable. Note: if
ClientHelloOuter and ClientHelloInner use different groups for their
key shares or differ in some other way, then the HelloRetryRequest
may actually be invalid for one or the other ClientHello, in which
case a fresh ClientHello MUST be generated, ignoring the instructions
in HelloRetryRequest. Otherwise, the usual rules for
HelloRetryRequest processing apply.
Clients bind encryption of the second ClientHelloInner to encryption
of the first ClientHelloInner via the derived ech_hrr_key by
modifying HPKE setup as follows:
pkR = Deserialize(ECHConfig.public_key)
enc, context = SetupPSKS(pkR, "tls ech" || 0x00 || ECHConfig,
ech_hrr_key, "hrr key")
The "info" parameter to SetupPSKS is the concatenation of "tls ech",
a zero byte, and the serialized ECHConfig. Clients then encrypt the
second ClientHelloInner using this new HPKE context. In doing so,
the encrypted value is also authenticated by ech_hrr_key. The
rationale for this is described in Section 10.10.2.
Client-facing servers perform the corresponding process when
decrypting second ClientHelloInner messages. In particular, upon
receipt of a second ClientHello message with a ClientECH value,
servers set up their HPKE context and decrypt ClientECH as follows:
context = SetupPSKR(ClientECH.enc, skR,
"tls ech" || 0x00 || ECHConfig, ech_hrr_key, "hrr key")
EncodedClientHelloInner = context.Open(ClientHelloOuterAAD,
ClientECH.payload)
ClientHelloOuterAAD is computed from the second ClientHelloOuter as
described in Section 5.2. The "info" parameter to SetupPSKR is
computed as above.
If the client offered ECH in the first ClientHello, then it MUST
offer ECH in the second. Likewise, if the client did not offer ECH
in the first ClientHello, then it MUST NOT not offer ECH in the
second.
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[[OPEN ISSUE: Should we be using the PSK input or the info input? On
the one hand, the requirements on info seem weaker, but maybe
actually this needs to be secret? Analysis needed.]]
6.4. GREASE Extensions
If the client attempts to connect to a server and does not have an
ECHConfig structure available for the server, it SHOULD send a GREASE
[RFC8701] "encrypted_client_hello" extension as follows:
* Set the "suite" field to a supported ECHCipherSuite. The
selection SHOULD vary to exercise all supported configurations,
but MAY be held constant for successive connections to the same
server in the same session.
* Set the "config_id" field to a randomly-generated string of "Nh"
bytes, where "Nh" is the output length of the "Extract" function
of the KDF associated with the chosen cipher suite. (The KDF API
is specified in [I-D.irtf-cfrg-hpke].)
* Set the "enc" field to a randomly-generated valid encapsulated
public key output by the HPKE KEM.
* Set the "payload" field to a randomly-generated string of L+C
bytes, where C is the ciphertext expansion of selected AEAD scheme
and L is the size of the ClientHelloInner message the client would
use given an ECHConfig structure, padded according to Section 6.2.
If the server sends an "encrypted_client_hello" extension, the client
MUST check the extension syntactically and abort the connection with
a "decode_error" alert if it is invalid. It otherwise ignores the
extension and MUST NOT use the retry keys.
[[OPEN ISSUE: if the client sends a GREASE "encrypted_client_hello"
extension, should it also send a GREASE "pre_shared_key" extension?
If not, GREASE+ticket is a trivial distinguisher.]]
Offering a GREASE extension is not considered offering an encrypted
ClientHello for purposes of requirements in Section 6. In
particular, the client MAY offer to resume sessions established
without ECH.
7. Server Behavior
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7.1. Client-Facing Server
Upon receiving an "encrypted_client_hello" extension, the client-
facing server determines if it will accept ECH, prior to negotiating
any other TLS parameters. Note that successfully decrypting the
extension will result in a new ClientHello to process, so even the
client's TLS version preferences may have changed.
First, the server collects a set of candidate ECHConfigs. This set
is determined by one of the two following methods:
1. Compare ClientECH.config_id against identifiers of known
ECHConfigs and select the one that matches, if any, as a
candidate.
2. Collect all known ECHConfigs as candidates, with trial decryption
below determining the final selection.
Some uses of ECH, such as local discovery mode, may omit the
ClientECH.config_id since it can be used as a tracking vector. In
such cases, the second method should be used for matching ClientECH
to known ECHConfig. See Section 10.4. Unless specified by the
application using (D)TLS or externally configured on both sides,
implementations MUST use the first method.
The server then iterates over all candidate ECHConfigs, attempting to
decrypt the "encrypted_client_hello" extension:
The server verifies that the ECHConfig supports the cipher suite
indicated by the ClientECH.cipher_suite and that the version of ECH
indicated by the client matches the ECHConfig.version. If not, the
server continues to the next candidate ECHConfig.
Next, the server decrypts ClientECH.payload, using the private key
skR corresponding to ECHConfig, as follows:
context = SetupBaseR(ClientECH.enc, skR,
"tls ech" || 0x00 || ECHConfig)
EncodedClientHelloInner = context.Open(ClientHelloOuterAAD,
ClientECH.payload)
ech_hrr_key = context.Export("tls ech hrr key", 32)
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ClientHelloOuterAAD is computed from ClientHelloOuter as described in
Section 5.2. The "info" parameter to SetupBaseS is the concatenation
"tls ech", a zero byte, and the serialized ECHConfig. If decryption
fails, the server continues to the next candidate ECHConfig.
Otherwise, the server reconstructs ClientHelloInner from
EncodedClientHelloInner, as described in Section 5.1. It then stops
consider candidate ECHConfigs.
Upon determining the ClientHelloInner, the client-facing server then
forwards the ClientHelloInner to the appropriate backend server,
which proceeds as in Section 7.2. If the backend server responds
with a HelloRetryRequest, the client-facing server forwards it,
decrypts the client's second ClientHelloOuter using the modified
procedure in Section 7.1.1, and forwards the resulting second
ClientHelloInner. The client-facing server forwards all other TLS
messages between the client and backend server unmodified.
Otherwise, if all candidate ECHConfigs fail to decrypt the extension,
the client-facing server MUST ignore the extension and proceed with
the connection using ClientHelloOuter. This connection proceeds as
usual, except the server MUST include the "encrypted_client_hello"
extension in its EncryptedExtensions with the "retry_configs" field
set to one or more ECHConfig structures with up-to-date keys.
Servers MAY supply multiple ECHConfig values of different versions.
This allows a server to support multiple versions at once.
Note that decryption failure could indicate a GREASE ECH extension
(see Section 6.4), so it is necessary for servers to proceed with the
connection and rely on the client to abort if ECH was required. In
particular, the unrecognized value alone does not indicate a
misconfigured ECH advertisement (Section 8.1). Instead, servers can
measure occurrences of the "ech_required" alert to detect this case.
7.1.1. HelloRetryRequest
In case a HelloRetryRequest (HRR) is sent, the client-facing server
MUST consistently accept or decline ECH between the two ClientHellos,
using the same ECHConfig, and abort the handshake if this is not
possible. This is achieved as follows. Let CH1 and CH2 denote,
respectively, the first and second ClientHello transmitted on the
wire by the client:
1. If CH1 contains the "encrypted_client_hello" extension but CH2
does not, or if CH2 contains the "encrypted_client_hello"
extension but CH1 does not, then the server MUST abort the
handshake with an "illegal_parameter" alert.
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2. If the "encrypted_client_hello" extension is sent in CH2, the
server follows the procedure in Section 7.1 to decrypt the
extension, but it uses the previously-selected ECHConfig as the
set of candidate ECHConfigs. If decryption fails, the server
aborts the connection with a "decrypt_error" alert rather than
continuing the handshake with the second ClientHelloOuter.
[[OPEN ISSUE: If the client-facing server implements stateless HRR,
it has no way to send a cookie, short of as-yet-unspecified
integration with the backend server. Stateful HRR on the client-
facing server works fine, however. See issue #333.]]
7.2. Backend Server Behavior
When the client-facing server accepts ECH, it forwards the
ClientHelloInner to the backend server, who terminates the
connection. If the ClientHelloInner contains an empty
"encrypted_client_hello" extension, then the backend server MUST
confirm ECH acceptance by setting ServerHello.random[24:32] to
accept_confirmation = HKDF-Expand-Label(
HKDF-Extract(0, ClientHelloInner.random),
"ech accept confirmation",
ServerHello.random[0:24], 8)
where HKDF-Expand-Label and HKDF-Extract are as defined in [RFC8446].
The value of ServerHello.random[0:24] is generated as usual by
invoking a secure random number generator (see [RFC8446],
Section 4.1.2).
8. Compatibility Issues
Unlike most TLS extensions, placing the SNI value in an ECH extension
is not interoperable with existing servers, which expect the value in
the existing plaintext extension. Thus server operators SHOULD
ensure servers understand a given set of ECH keys before advertising
them. Additionally, servers SHOULD retain support for any
previously-advertised keys for the duration of their validity
However, in more complex deployment scenarios, this may be difficult
to fully guarantee. Thus this protocol was designed to be robust in
case of inconsistencies between systems that advertise ECH keys and
servers, at the cost of extra round-trips due to a retry. Two
specific scenarios are detailed below.
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8.1. Misconfiguration and Deployment Concerns
It is possible for ECH advertisements and servers to become
inconsistent. This may occur, for instance, from DNS
misconfiguration, caching issues, or an incomplete rollout in a
multi-server deployment. This may also occur if a server loses its
ECH keys, or if a deployment of ECH must be rolled back on the
server.
The retry mechanism repairs inconsistencies, provided the server is
authoritative for the public name. If server and advertised keys
mismatch, the server will respond with ech_retry_requested. If the
server does not understand the "encrypted_client_hello" extension at
all, it will ignore it as required by [RFC8446]; Section 4.1.2.
Provided the server can present a certificate valid for the public
name, the client can safely retry with updated settings, as described
in Section 6.3.
Unless ECH is disabled as a result of successfully establishing a
connection to the public name, the client MUST NOT fall back to using
unencrypted ClientHellos, as this allows a network attacker to
disclose the contents of this ClientHello, including the SNI. It MAY
attempt to use another server from the DNS results, if one is
provided.
8.2. Middleboxes
A more serious problem is MITM proxies which do not support this
extension. [RFC8446], Section 9.3 requires that such proxies remove
any extensions they do not understand. The handshake will then
present a certificate based on the public name, without echoing the
"encrypted_client_hello" extension to the client.
Depending on whether the client is configured to accept the proxy's
certificate as authoritative for the public name, this may trigger
the retry logic described in Section 6.3 or result in a connection
failure. A proxy which is not authoritative for the public name
cannot forge a signal to disable ECH.
A non-conformant MITM proxy which instead forwards the ECH extension,
substituting its own KeyShare value, will result in the client-facing
server recognizing the key, but failing to decrypt the SNI. This
causes a hard failure. Clients SHOULD NOT attempt to repair the
connection in this case.
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9. Compliance Requirements
In the absence of an application profile standard specifying
otherwise, a compliant ECH application MUST implement the following
HPKE cipher suite:
* KEM: DHKEM(X25519, HKDF-SHA256) (see [I-D.irtf-cfrg-hpke],
Section 7.1)
* KDF: HKDF-SHA256 (see [I-D.irtf-cfrg-hpke], Section 7.2)
* AEAD: AES-128-GCM (see [I-D.irtf-cfrg-hpke], Section 7.3)
10. Security Considerations
10.1. Security and Privacy Goals
ECH considers two types of attackers: passive and active. Passive
attackers can read packets from the network. They cannot perform any
sort of active behavior such as probing servers or querying DNS. A
middlebox that filters based on plaintext packet contents is one
example of a passive attacker. In contrast, active attackers can
write packets into the network for malicious purposes, such as
interfering with existing connections, probing servers, and querying
DNS. In short, an active attacker corresponds to the conventional
threat model for TLS 1.3 [RFC8446].
Given these types of attackers, the primary goals of ECH are as
follows.
1. Use of ECH does not weaken the security properties of TLS without
ECH.
2. TLS connection establishment to a host with a specific ECHConfig
and TLS configuration is indistinguishable from a connection to
any other host with the same ECHConfig and TLS configuration.
(The set of hosts which share the same ECHConfig and TLS
configuration is referred to as the anonymity set.)
Client-facing server configuration determines the size of the
anonymity set. For example, if a client-facing server uses distinct
ECHConfig values for each host, then each anonymity set has size k =
1. Client-facing servers SHOULD deploy ECH in such a way so as to
maximize the size of the anonymity set where possible. This means
client-facing servers should use the same ECHConfig for as many hosts
as possible. An attacker can distinguish two hosts that have
different ECHConfig values based on the ClientECH.config_id value.
This also means public information in a TLS handshake is also
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consistent across hosts. For example, if a client-facing server
services many backend origin hosts, only one of which supports some
cipher suite, it may be possible to identify that host based on the
contents of unencrypted handshake messages.
Beyond these primary security and privacy goals, ECH also aims to
hide, to some extent, (a) whether or not a specific server supports
ECH and (b) whether or not ECH was accepted for a particular
connection. ECH aims to achieve both properties, assuming the
attacker is passive and does not know the set of ECH configurations
offered by the client-facing server. It does not achieve these
properties for active attackers. More specifically:
* Passive attackers with a known ECH configuration can distinguish
between a connection that negotiates ECH with that configuration
and one which does not, because the latter used a GREASE
"encrypted_client_hello" extension (as specified in Section 6.4)
or a different ECH configuration.
* Passive attackers without the ECH configuration cannot distinguish
between a connection that negotiates ECH and one which uses a
GREASE "encrypted_client_hello" extension.
* Active attackers can distinguish between a connection that
negotiates ECH and one which uses a GREASE
"encrypted_client_hello" extension.
See Section 10.8.4 for more discussion about the "do not stick out"
criteria from [RFC8744].
10.2. Unauthenticated and Plaintext DNS
In comparison to [I-D.kazuho-protected-sni], wherein DNS Resource
Records are signed via a server private key, ECH records have no
authenticity or provenance information. This means that any attacker
which can inject DNS responses or poison DNS caches, which is a
common scenario in client access networks, can supply clients with
fake ECH records (so that the client encrypts data to them) or strip
the ECH record from the response. However, in the face of an
attacker that controls DNS, no encryption scheme can work because the
attacker can replace the IP address, thus blocking client
connections, or substituting a unique IP address which is 1:1 with
the DNS name that was looked up (modulo DNS wildcards). Thus,
allowing the ECH records in the clear does not make the situation
significantly worse.
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Clearly, DNSSEC (if the client validates and hard fails) is a defense
against this form of attack, but DoH/DPRIVE are also defenses against
DNS attacks by attackers on the local network, which is a common case
where ClientHello and SNI encryption are desired. Moreover, as noted
in the introduction, SNI encryption is less useful without encryption
of DNS queries in transit via DoH or DPRIVE mechanisms.
10.3. Client Tracking
A malicious client-facing server could distribute unique, per-client
ECHConfig structures as a way of tracking clients across subsequent
connections. On-path adversaries which know about these unique keys
could also track clients in this way by observing TLS connection
attempts.
The cost of this type of attack scales linearly with the desired
number of target clients. Moreover, DNS caching behavior makes
targeting individual users for extended periods of time, e.g., using
per-client ECHConfig structures delivered via HTTPS RRs with high
TTLs, challenging. Clients can help mitigate this problem by
flushing any DNS or ECHConfig state upon changing networks.
10.4. Optional Configuration Identifiers and Trial Decryption
Optional configuration identifiers may be useful in scenarios where
clients and client-facing servers do not want to reveal information
about the client-facing server in the "encrypted_client_hello"
extension. In such settings, clients send either an empty config_id
or a randomly generated config_id in the ClientECH. (The precise
implementation choice for this mechanism is out of scope for this
document.) Servers in these settings must perform trial decryption
since they cannot identify the client's chosen ECH key using the
config_id value. As a result, support for optional configuration
identifiers may exacerbate DoS attacks. Specifically, an adversary
may send malicious ClientHello messages, i.e., those which will not
decrypt with any known ECH key, in order to force wasteful
decryption. Servers that support this feature should, for example,
implement some form of rate limiting mechanism to limit the damage
caused by such attacks.
10.5. Outer ClientHello
Any information that the client includes in the ClientHelloOuter is
visible to passive observers. The client SHOULD NOT send values in
the ClientHelloOuter which would reveal a sensitive ClientHelloInner
property, such as the true server name. It MAY send values
associated with the public name in the ClientHelloOuter.
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In particular, some extensions require the client send a server-name-
specific value in the ClientHello. These values may reveal
information about the true server name. For example, the
"cached_info" ClientHello extension [RFC7924] can contain the hash of
a previously observed server certificate. The client SHOULD NOT send
values associated with the true server name in the ClientHelloOuter.
It MAY send such values in the ClientHelloInner.
A client may also use different preferences in different contexts.
For example, it may send a different ALPN lists to different servers
or in different application contexts. A client that treats this
context as sensitive SHOULD NOT send context-specific values in
ClientHelloOuter.
Values which are independent of the true server name, or other
information the client wishes to protect, MAY be included in
ClientHelloOuter. If they match the corresponding ClientHelloInner,
they MAY be compressed as described in Section 5.1. However, note
the payload length reveals information about which extensions are
compressed, so inner extensions which only sometimes match the
corresponding outer extension SHOULD NOT be compressed.
Clients MAY include additional extensions in ClientHelloOuter to
avoid signaling unusual behavior to passive observers, provided the
choice of value and value itself are not sensitive. See
Section 10.8.4.
10.6. Related Privacy Leaks
ECH requires encrypted DNS to be an effective privacy protection
mechanism. However, verifying the server's identity from the
Certificate message, particularly when using the X509
CertificateType, may result in additional network traffic that may
reveal the server identity. Examples of this traffic may include
requests for revocation information, such as OCSP or CRL traffic, or
requests for repository information, such as
authorityInformationAccess. It may also include implementation-
specific traffic for additional information sources as part of
verification.
Implementations SHOULD avoid leaking information that may identify
the server. Even when sent over an encrypted transport, such
requests may result in indirect exposure of the server's identity,
such as indicating a specific CA or service being used. To mitigate
this risk, servers SHOULD deliver such information in-band when
possible, such as through the use of OCSP stapling, and clients
SHOULD take steps to minimize or protect such requests during
certificate validation.
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10.7. Attacks Exploiting Acceptance Confirmation
To signal acceptance, the backend server overwrites 8 bytes of its
ServerHello.random with a value derived from the
ClientHelloInner.random. (See Section 7.2 for details.) This
behavior increases the likelihood of the ServerHello.random colliding
with the ServerHello.random of a previous session, potentially
reducing the overall security of the protocol. However, the
remaining 24 bytes provide enough entropy to ensure this is not a
practical avenue of attack.
On the other hand, the probability that two 8-byte strings are the
same is non-negligible. This poses a modest operational risk.
Suppose the client-facing server terminates the connection (i.e., ECH
is rejected or bypassed): if the last 8 bytes of its
ServerHello.random coincide with the confirmation signal, then the
client will incorrectly presume acceptance and proceed as if the
backend server terminated the connection. However, the probability
of a false positive occurring for a given connection is only 1 in
2^64. This value is smaller than the probability of network
connection failures in practice.
Note that the same bytes of the ServerHello.random are used to
implement downgrade protection for TLS 1.3 (see [RFC8446],
Section 4.1.3). The backend server's signal of acceptance does not
interfere with this mechanism because ECH is only supported in TLS
1.3 or higher.
10.8. Comparison Against Criteria
[RFC8744] lists several requirements for SNI encryption. In this
section, we re-iterate these requirements and assess the ECH design
against them.
10.8.1. Mitigate Cut-and-Paste Attacks
Since servers process either ClientHelloInner or ClientHelloOuter,
and because ClientHelloInner.random is encrypted, it is not possible
for an attacker to "cut and paste" the ECH value in a different
Client Hello and learn information from ClientHelloInner.
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10.8.2. Avoid Widely Shared Secrets
This design depends upon DNS as a vehicle for semi-static public key
distribution. Server operators may partition their private keys
however they see fit provided each server behind an IP address has
the corresponding private key to decrypt a key. Thus, when one ECH
key is provided, sharing is optimally bound by the number of hosts
that share an IP address. Server operators may further limit sharing
by publishing different DNS records containing ECHConfig values with
different keys using a short TTL.
10.8.3. Prevent SNI-Based Denial-of-Service Attacks
This design requires servers to decrypt ClientHello messages with
ClientECH extensions carrying valid digests. Thus, it is possible
for an attacker to force decryption operations on the server. This
attack is bound by the number of valid TCP connections an attacker
can open.
10.8.4. Do Not Stick Out
The only explicit signal indicating possible use of ECH is the
ClientHello "encrypted_client_hello" extension. Server handshake
messages do not contain any signal indicating use or negotiation of
ECH. Clients MAY GREASE the "encrypted_client_hello" extension, as
described in Section 6.4, which helps ensure the ecosystem handles
ECH correctly. Moreover, as more clients enable ECH support, e.g.,
as normal part of Web browser functionality, with keys supplied by
shared hosting providers, the presence of ECH extensions becomes less
unusual and part of typical client behavior. In other words, if all
Web browsers start using ECH, the presence of this value will not
signal unusual behavior to passive eavesdroppers.
10.8.5. Maintain Forward Secrecy
This design is not forward secret because the server's ECH key is
static. However, the window of exposure is bound by the key
lifetime. It is RECOMMENDED that servers rotate keys frequently.
10.8.6. Enable Multi-party Security Contexts
This design permits servers operating in Split Mode to forward
connections directly to backend origin servers. The client
authenticates the identity of the backend origin server, thereby
avoiding unnecessary MiTM attacks.
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Conversely, assuming ECH records retrieved from DNS are
authenticated, e.g., via DNSSEC or fetched from a trusted Recursive
Resolver, spoofing a client-facing server operating in Split Mode is
not possible. See Section 10.2 for more details regarding plaintext
DNS.
Authenticating the ECHConfigs structure naturally authenticates the
included public name. This also authenticates any retry signals from
the client-facing server because the client validates the server
certificate against the public name before retrying.
10.8.7. Support Multiple Protocols
This design has no impact on application layer protocol negotiation.
It may affect connection routing, server certificate selection, and
client certificate verification. Thus, it is compatible with
multiple application and transport protocols. By encrypting the
entire ClientHello, this design additionally supports encrypting the
ALPN extension.
10.9. Padding Policy
Variations in the length of the ClientHelloInner ciphertext could
leak information about the corresponding plaintext. Section 6.2
describes a RECOMMENDED padding mechanism for clients aimed at
reducing potential information leakage.
10.10. Active Attack Mitigations
This section describes the rationale for ECH properties and mechanics
as defenses against active attacks. In all the attacks below, the
attacker is on-path between the target client and server. The goal
of the attacker is to learn private information about the inner
ClientHello, such as the true SNI value.
10.10.1. Client Reaction Attack Mitigation
This attack uses the client's reaction to an incorrect certificate as
an oracle. The attacker intercepts a legitimate ClientHello and
replies with a ServerHello, Certificate, CertificateVerify, and
Finished messages, wherein the Certificate message contains a "test"
certificate for the domain name it wishes to query. If the client
decrypted the Certificate and failed verification (or leaked
information about its verification process by a timing side channel),
the attacker learns that its test certificate name was incorrect. As
an example, suppose the client's SNI value in its inner ClientHello
is "example.com," and the attacker replied with a Certificate for
"test.com". If the client produces a verification failure alert
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because of the mismatch faster than it would due to the Certificate
signature validation, information about the name leaks. Note that
the attacker can also withhold the CertificateVerify message. In
that scenario, a client which first verifies the Certificate would
then respond similarly and leak the same information.
Client Attacker Server
ClientHello
+ key_share
+ ech ------> (intercept) -----> X (drop)
ServerHello
+ key_share
{EncryptedExtensions}
{CertificateRequest*}
{Certificate*}
{CertificateVerify*}
<------
Alert
------>
Figure 3: Client reaction attack
ClientHelloInner.random prevents this attack. In particular, since
the attacker does not have access to this value, it cannot produce
the right transcript and handshake keys needed for encrypting the
Certificate message. Thus, the client will fail to decrypt the
Certificate and abort the connection.
10.10.2. HelloRetryRequest Hijack Mitigation
This attack aims to exploit server HRR state management to recover
information about a legitimate ClientHello using its own attacker-
controlled ClientHello. To begin, the attacker intercepts and
forwards a legitimate ClientHello with an "encrypted_client_hello"
(ech) extension to the server, which triggers a legitimate
HelloRetryRequest in return. Rather than forward the retry to the
client, the attacker, attempts to generate its own ClientHello in
response based on the contents of the first ClientHello and
HelloRetryRequest exchange with the result that the server encrypts
the Certificate to the attacker. If the server used the SNI from the
first ClientHello and the key share from the second (attacker-
controlled) ClientHello, the Certificate produced would leak the
client's chosen SNI to the attacker.
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Client Attacker Server
ClientHello
+ key_share
+ ech ------> (forward) ------->
HelloRetryRequest
+ key_share
(intercept) <-------
ClientHello
+ key_share'
+ ech' ------->
ServerHello
+ key_share
{EncryptedExtensions}
{CertificateRequest*}
{Certificate*}
{CertificateVerify*}
{Finished}
<-------
(process server flight)
Figure 4: HelloRetryRequest hijack attack
This attack is mitigated by binding the first and second ClientHello
messages together. In particular, since the attacker does not
possess the ech_hrr_key, it cannot generate a valid encryption of the
second inner ClientHello. The server will attempt decryption using
ech_hrr_key, detect failure, and fail the connection.
If the second ClientHello were not bound to the first, it might be
possible for the server to act as an oracle if it required parameters
from the first ClientHello to match that of the second ClientHello.
For example, imagine the client's original SNI value in the inner
ClientHello is "example.com", and the attacker's hijacked SNI value
in its inner ClientHello is "test.com". A server which checks these
for equality and changes behavior based on the result can be used as
an oracle to learn the client's SNI.
10.10.3. ClientHello Malleability Mitigation
This attack aims to leak information about secret parts of the
encrypted ClientHello by adding attacker-controlled parameters and
observing the server's response. In particular, the compression
mechanism described in Section 5.1 references parts of a potentially
attacker-controlled ClientHelloOuter to construct ClientHelloInner,
or a buggy server may incorrectly apply parameters from
ClientHelloOuter to the handshake.
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To begin, the attacker first interacts with a server to obtain a
resumption ticket for a given test domain, such as "example.com".
Later, upon receipt of a ClientHelloOuter, it modifies it such that
the server will process the resumption ticket with ClientHelloInner.
If the server only accepts resumption PSKs that match the server
name, it will fail the PSK binder check with an alert when
ClientHelloInner is for "example.com" but silently ignore the PSK and
continue when ClientHelloInner is for any other name. This
introduces an oracle for testing encrypted SNI values.
Client Attacker Server
handshake and ticket
for "example.com"
<-------->
ClientHello
+ key_share
+ ech
+ outer_extensions(pre_shared_key)
+ pre_shared_key
-------->
(intercept)
ClientHello
+ key_share
+ ech
+ outer_extensions(pre_shared_key)
+ pre_shared_key'
-------->
Alert
-or-
ServerHello
...
Finished
<--------
Figure 5: Message flow for malleable ClientHello
This attack may be generalized to any parameter which the server
varies by server name, such as ALPN preferences.
ECH mitigates this attack by only negotiating TLS parameters from
ClientHelloInner and authenticating all inputs to the
ClientHelloInner (EncodedClientHelloInner and ClientHelloOuter) with
the HPKE AEAD. See Section 5.2. An earlier iteration of this
specification only encrypted and authenticated the "server_name"
extension, which left the overall ClientHello vulnerable to an
analogue of this attack.
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11. IANA Considerations
11.1. Update of the TLS ExtensionType Registry
IANA is requested to create the following two entries in the existing
registry for ExtensionType (defined in [RFC8446]):
1. encrypted_client_hello(0xfe08), with "TLS 1.3" column values
being set to "CH, EE", and "Recommended" column being set to
"Yes".
2. outer_extensions(0xfd00), with the "TLS 1.3" column values being
set to "", and "Recommended" column being set to "Yes".
11.2. Update of the TLS Alert Registry
IANA is requested to create an entry, ech_required(121) in the
existing registry for Alerts (defined in [RFC8446]), with the "DTLS-
OK" column being set to "Y".
12. ECHConfig Extension Guidance
Any future information or hints that influence ClientHelloOuter
SHOULD be specified as ECHConfig extensions. This is primarily
because the outer ClientHello exists only in support of ECH. Namely,
it is both an envelope for the encrypted inner ClientHello and
enabler for authenticated key mismatch signals (see Section 7). In
contrast, the inner ClientHello is the true ClientHello used upon ECH
negotiation.
13. References
13.1. Normative References
[HTTPS-RR] Schwartz, B., Bishop, M., and E. Nygren, "Service binding
and parameter specification via the DNS (DNS SVCB and
HTTPS RRs)", Work in Progress, Internet-Draft, draft-ietf-
dnsop-svcb-https-01, 13 July 2020, <http://www.ietf.org/
internet-drafts/draft-ietf-dnsop-svcb-https-01.txt>.
[I-D.ietf-tls-exported-authenticator]
Sullivan, N., "Exported Authenticators in TLS", Work in
Progress, Internet-Draft, draft-ietf-tls-exported-
authenticator-13, 26 June 2020, <http://www.ietf.org/
internet-drafts/draft-ietf-tls-exported-authenticator-
13.txt>.
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[I-D.irtf-cfrg-hpke]
Barnes, R., Bhargavan, K., Lipp, B., and C. Wood, "Hybrid
Public Key Encryption", Work in Progress, Internet-Draft,
draft-irtf-cfrg-hpke-05, 30 July 2020,
<http://www.ietf.org/internet-drafts/draft-irtf-cfrg-hpke-
05.txt>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC7685] Langley, A., "A Transport Layer Security (TLS) ClientHello
Padding Extension", RFC 7685, DOI 10.17487/RFC7685,
October 2015, <https://www.rfc-editor.org/info/rfc7685>.
[RFC7918] Langley, A., Modadugu, N., and B. Moeller, "Transport
Layer Security (TLS) False Start", RFC 7918,
DOI 10.17487/RFC7918, August 2016,
<https://www.rfc-editor.org/info/rfc7918>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
13.2. Informative References
[I-D.kazuho-protected-sni]
Oku, K., "TLS Extensions for Protecting SNI", Work in
Progress, Internet-Draft, draft-kazuho-protected-sni-00,
18 July 2017, <http://www.ietf.org/internet-drafts/draft-
kazuho-protected-sni-00.txt>.
[RFC7301] Friedl, S., Popov, A., Langley, A., and E. Stephan,
"Transport Layer Security (TLS) Application-Layer Protocol
Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
July 2014, <https://www.rfc-editor.org/info/rfc7301>.
[RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
and P. Hoffman, "Specification for DNS over Transport
Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
2016, <https://www.rfc-editor.org/info/rfc7858>.
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[RFC7924] Santesson, S. and H. Tschofenig, "Transport Layer Security
(TLS) Cached Information Extension", RFC 7924,
DOI 10.17487/RFC7924, July 2016,
<https://www.rfc-editor.org/info/rfc7924>.
[RFC8094] Reddy, T., Wing, D., and P. Patil, "DNS over Datagram
Transport Layer Security (DTLS)", RFC 8094,
DOI 10.17487/RFC8094, February 2017,
<https://www.rfc-editor.org/info/rfc8094>.
[RFC8484] Hoffman, P. and P. McManus, "DNS Queries over HTTPS
(DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
<https://www.rfc-editor.org/info/rfc8484>.
[RFC8701] Benjamin, D., "Applying Generate Random Extensions And
Sustain Extensibility (GREASE) to TLS Extensibility",
RFC 8701, DOI 10.17487/RFC8701, January 2020,
<https://www.rfc-editor.org/info/rfc8701>.
[RFC8744] Huitema, C., "Issues and Requirements for Server Name
Identification (SNI) Encryption in TLS", RFC 8744,
DOI 10.17487/RFC8744, July 2020,
<https://www.rfc-editor.org/info/rfc8744>.
Appendix A. Alternative SNI Protection Designs
Alternative approaches to encrypted SNI may be implemented at the TLS
or application layer. In this section we describe several
alternatives and discuss drawbacks in comparison to the design in
this document.
A.1. TLS-layer
A.1.1. TLS in Early Data
In this variant, TLS Client Hellos are tunneled within early data
payloads belonging to outer TLS connections established with the
client-facing server. This requires clients to have established a
previous session --- and obtained PSKs --- with the server. The
client-facing server decrypts early data payloads to uncover Client
Hellos destined for the backend server, and forwards them onwards as
necessary. Afterwards, all records to and from backend servers are
forwarded by the client-facing server - unmodified. This avoids
double encryption of TLS records.
Problems with this approach are: (1) servers may not always be able
to distinguish inner Client Hellos from legitimate application data,
(2) nested 0-RTT data may not function correctly, (3) 0-RTT data may
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not be supported - especially under DoS - leading to availability
concerns, and (4) clients must bootstrap tunnels (sessions), costing
an additional round trip and potentially revealing the SNI during the
initial connection. In contrast, encrypted SNI protects the SNI in a
distinct Client Hello extension and neither abuses early data nor
requires a bootstrapping connection.
A.1.2. Combined Tickets
In this variant, client-facing and backend servers coordinate to
produce "combined tickets" that are consumable by both. Clients
offer combined tickets to client-facing servers. The latter parse
them to determine the correct backend server to which the Client
Hello should be forwarded. This approach is problematic due to non-
trivial coordination between client-facing and backend servers for
ticket construction and consumption. Moreover, it requires a
bootstrapping step similar to that of the previous variant. In
contrast, encrypted SNI requires no such coordination.
A.2. Application-layer
A.2.1. HTTP/2 CERTIFICATE Frames
In this variant, clients request secondary certificates with
CERTIFICATE_REQUEST HTTP/2 frames after TLS connection completion.
In response, servers supply certificates via TLS exported
authenticators [I-D.ietf-tls-exported-authenticator] in CERTIFICATE
frames. Clients use a generic SNI for the underlying client-facing
server TLS connection. Problems with this approach include: (1) one
additional round trip before peer authentication, (2) non-trivial
application-layer dependencies and interaction, and (3) obtaining the
generic SNI to bootstrap the connection. In contrast, encrypted SNI
induces no additional round trip and operates below the application
layer.
Appendix B. Acknowledgements
This document draws extensively from ideas in
[I-D.kazuho-protected-sni], but is a much more limited mechanism
because it depends on the DNS for the protection of the ECH key.
Richard Barnes, Christian Huitema, Patrick McManus, Matthew Prince,
Nick Sullivan, Martin Thomson, and David Benjamin also provided
important ideas and contributions.
Authors' Addresses
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Eric Rescorla
RTFM, Inc.
Email: ekr@rtfm.com
Kazuho Oku
Fastly
Email: kazuhooku@gmail.com
Nick Sullivan
Cloudflare
Email: nick@cloudflare.com
Christopher A. Wood
Cloudflare
Email: caw@heapingbits.net
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